The invention relates to communications systems and methods, and more particularly to a system and method for realizing high power efficiency in wireless transmission and effective control of transmission power.
It is well known that with no excitation applied to an amplifier, an operating point, known as a xe2x80x9cquiescent pointxe2x80x9d or xe2x80x9cDC operating point,xe2x80x9d is defined in an active region of the output characteristics of the amplifier. To provide linear amplification for a radio-frequency (RF) input signal for example, a class A amplifier is normally used, which is typically biased to place the DC operating point far enough from both cutoff and saturation regions in its output characteristics. As a result, the input signal excursions to either side of the operating point do not cause the signal to be cut off or the amplifier to be saturated, thus avoiding introducing distortion into the signal. However, a major drawback of the class A amplifier is its low power efficiency due to a relatively high DC input power required by the amplifier, with respect to its output signal power.
A class B amplifier is typically biased to cut off a half-cycle of an input signal, with an output current flow only during the positive half-cycle of the signal. As a result, the amplifier output is significantly distorted, with respect to the input signal. Thus, a class B amplifier is unsuitable for a typical linear operation. However, with respect to a class A amplifier, a class B amplifier affords higher power efficiency as the DC input power to the amplifier is relatively low.
A class AB amplifier, on the other hand, is typically biased in such a way that the output current flows for more than half of the cycle of the input signal. As a result, a class AB amplifier behaves like a hybrid between the class A and class B amplifiers. Thus, a class AB amplifier causes a lower distortion than a class B amplifier (but a higher distortion than a class A amplifier) to an input signal at a high power level. At the same time, the class AB amplifier realizes higher power efficiency than a class A amplifier.
A class C amplifier is typically biased in such a way that the output current flows less than half of the cycle of the input signal. As a result, the class C amplifier affords the highest power efficiency and, unfortunately, also distortion to an input signal of all of the aforementioned amplifiers.
A power amplifier is typically used in a transmitter of a digital wireless communications system, e.g., personal communications service (PCS) system, to boost the power level of a digitally modulated signal for transmission thereof. In wireless communications, the power amplifier is required to afford linear amplification to the signal, without introducing significant distortion thereinto. As such, in the prior art, the power amplifier used in a wireless transmitter typically operates in a class A and/or class AB mode to satisfy the linear amplification requirement.
However, for example, a transmitter in a wireless telephone handset is normally powered by a rechargeable battery having a limited capacity. The usable transmission time allowed by the battery before recharging thereof increases with the power efficiency afforded by the power amplifier in the transmitter. Since a long allowable transmission time, and accordingly high amplifier power efficiency, is always desired, the prior art power amplifier operating in a class A and/or class AB mode is deficient in that it affords relatively low power efficiency, compared with a class B or class C amplifier.
Nevertheless, it is well known that where a class B or class C amplifier is used, a feedback mechanism may be employed to xe2x80x9clinearizexe2x80x9d the amplifier to afford substantially linear amplification.
It appears that a class B or class C amplifier can be used as the power amplifier in the prior art wireless communications system to increase the power efficiency, in conjunction with the aforementioned feedback mechanism which helps linearize the class B or class C amplifier. However, I have recognized that in such an arrangement, because of the feedback mechanism, a gain control in the prior art wireless communications system which is normally used for controlling the transmission power level can no longer be effectively used for that purpose. Such ineffective power control is particularly disadvantageous in wireless communications as the transmission power requirement frequently changes.
In accordance with the invention, a second gain control is incorporated in the above feedback mechanism to impart a selected gain in a feedback version of a transmitted signal, thereby effectively control an overall gain, and thus the power level, of the transmitted signal. A difference between a phase of the transmitted signal and that of the feedback version thereof is also reduced to increase the linearity of the power amplifier and stabilize the feedback mechanism.